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BBS5
Bardet-Biedl syndrome 5
BBS5 is located on the long (q) arm of chromosome 2, at band 2q31.1. Arm ratio per GRCh38 - banding schematic.
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Overview
BBS5 (Bardet-Biedl syndrome 5) encodes a component of the BBSome, an octameric protein complex localised to primary cilia. Primary cilia are microscopic structures that extend from the surface of most mammalian cells, functioning as signalling hubs that detect extracellular cues and regulate developmental pathways. The BBS5 protein works alongside seven other BBSome subunits to ensure proper assembly and trafficking of proteins within cilia.
Pathogenic variants in BBS5 follow an autosomal recessive inheritance pattern, meaning that individuals typically require changes in both gene copies to develop Bardet-Biedl syndrome. This multisystem ciliopathy presents with a constellation of features including retinal degeneration, obesity, kidney abnormalities, polydactyly, and developmental differences. The gene is located on chromosome 2 and produces a protein of 341 amino acids.
What the gene does
The BBS5 protein functions as a core structural component of the BBSome complex, which serves as an adaptor between cargo proteins and intraflagellar transport machinery within primary cilia. This complex mediates the selective entry and exit of signalling receptors and membrane proteins at the ciliary gate, thereby regulating which molecules access the ciliary compartment.
Within the BBSome, BBS5 contributes to the overall stability of the octameric assembly. The complex coordinates several critical signalling pathways, including Hedgehog signalling during embryonic development and leptin receptor trafficking in hypothalamic neurons. Disruption of BBSome function impairs the cell's ability to respond appropriately to growth factors and hormones, affecting tissue development and metabolic regulation.
The protein also participates in regulating the ciliary localisation of G protein-coupled receptors. By controlling receptor distribution between the ciliary membrane and the cell body, BBS5 and its BBSome partners help cells interpret environmental signals correctly. This regulatory role is particularly important in photoreceptor cells of the retina and in cells lining kidney tubules.
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Chromosome location
BBS5 is located on the long arm of chromosome 2 at position 31.1 (2q31.1). The gene spans approximately 70 kilobases of genomic DNA and contains multiple exons that encode the 341-amino-acid protein. This chromosomal region contains several other genes involved in developmental processes, though BBS5 itself is a distinct transcriptional unit with its own regulatory elements.
Protein structure
Domain architecture has not been experimentally characterised in detail for this protein. The BBS5 polypeptide folds into a structure that enables interaction with other BBSome components, particularly BBS1, BBS2, BBS4, BBS7, BBS8, BBS9, and BBIP10. Structural studies of the assembled BBSome complex suggest that BBS5 contributes to the overall scaffold architecture, though specific functional domains within the BBS5 sequence remain to be fully defined at the molecular level.
Key variants
Pathogenic variants in BBS5 are distributed throughout the coding sequence, with both missense changes and truncating variants reported in individuals with Bardet-Biedl syndrome. The variant spectrum includes nonsense mutations that introduce premature stop codons, frameshift variants arising from small insertions or deletions, and missense substitutions that disrupt protein folding or interaction interfaces. Certain populations show founder effects, with specific pathogenic variants occurring at higher frequencies due to shared ancestry.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.123del | p.Gly42fs | Pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.143-1G>C | - | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome 5 |
c.209-1G>A | - | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome 5 |
c.303dup | p.Asn102Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.425T>G | p.Leu142Ter | Pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.444del | p.Asn149fs | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.567G>A | p.Trp189Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome 5 |
c.681+1G>T | - | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome 5 |
c.709del | p.Lys236_Ile237insTer | Pathogenic | ★★☆☆ | Bardet-Biedl syndrome 5 |
c.944_960del | p.Val315fs | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Pathogenic variants in BBS5 cause Bardet-Biedl syndrome, a rare ciliopathy characterised by retinal dystrophy leading to progressive vision loss, early-onset obesity, polydactyly, renal anomalies, learning difficulties, and hypogonadism. The clinical presentation is variable, with individuals showing different combinations and severities of these features. Diagnosis typically involves clinical evaluation combined with molecular genetic testing, as the overlap of features among different BBS genes means that genotype-phenotype correlations are limited. Some individuals may present with incomplete or atypical features, particularly in childhood before the full syndrome becomes apparent.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic BBS5 variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
BBS5 holds green classification status across multiple NHS Genomic Medicine Service panels, reflecting strong clinical and research evidence supporting its role in human disease. The gene appears on the Bardet-Biedl syndrome panel (R107), reflecting its established role in this condition. It is also included in panels for foetal anomalies (R21), intellectual disability (R29), retinal disorders (R32), severe early-onset obesity (R149), and skeletal dysplasia (R104). Additional memberships include the DDG2P database, ophthalmological ciliopathies, rare multisystem ciliopathy disorders, renal ciliopathies, and limb disorders panels. This broad panel representation reflects the multisystem nature of ciliopathies and ensures appropriate genetic testing pathways for individuals presenting with relevant clinical features.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the inheritance pattern for BBS5-related conditions?
BBS5-related Bardet-Biedl syndrome follows an autosomal recessive inheritance pattern. This means that an individual typically needs to inherit a pathogenic variant from both parents to develop the condition. Carriers with one variant copy generally do not show symptoms.
How does BBS5 relate to ciliary function?
The BBS5 protein is a core component of the BBSome, a complex that regulates protein trafficking within primary cilia. These cellular antennae coordinate signalling pathways essential for development and sensory processes. When BBS5 function is disrupted, ciliary signalling becomes impaired across multiple tissues.
Why does BBS5 testing appear on multiple NHS panels?
Bardet-Biedl syndrome is a multisystem disorder affecting the eyes, kidneys, limbs, metabolism, and cognition. BBS5 appears on numerous NHS panels because individuals may present initially with isolated features such as retinal disease, obesity, or developmental differences before the full syndrome becomes apparent.